In use of plural battery cells that are connected in series, variation in voltages (capacities) thereof occurs according to a characteristic of each battery cell, a use environment, or the like. The variation of the voltages may cause over-charge or over-discharge. Further, when plural battery cells that are connected in series are used, charging is stopped at a voltage such that a battery cell that has the highest voltage is not over-charged during charging, and discharging is stopped at a voltage such that a battery cell that has the lowest voltage is not over-discharged during discharging. Thus, a use range of the entire series voltages becomes limited, and thus, usable capacities of the battery cells are reduced.
As an example of a technique that solves such a problem, there is a charge state control device of a battery pack disclosed in Patent Document 1. The charge state control device of the battery pack disclosed in Patent Document 1 includes plural battery cell groups including plural battery cells that are connected in series, plural capacitors, plural voltmeters, plural switching circuits respectively configured of plural switches, and a control circuit, in which a part of the battery cells belongs to both of two adjacent battery cell groups. With this configuration, the charge state control device disclosed in Patent Document 1 achieves uniformization of a charge state in each battery cell group and uniformization of a charge state between the respective battery cell groups.